The present invention relates to a silicone rubber sponge composition, to a silicone rubber sponge, and to a process for production thereof. More particularly, it relates to a silicone rubber sponge composition that gives a silicone rubber sponge having fine, uniform cells, to a silicone rubber sponge, and to a process for production thereof.
Due to their outstanding heat and weather resistance and light weight, silicone rubber sponges are used for automotive parts, such as packings, gaskets, and O-rings; sheath materials for rollers in copiers; gaskets for construction use such as joint fillers and sealers; and various sealing and cushioning applications. A number of silicone rubber sponge compositions have been proposed to date. Patent Publication 44-461 and Patent Application Laying Open 7-247436, for example, teach heat curing silicone rubber sponge compositions containing thermally decomposable organic blowing agents such as azobisisobutyronitrile. However, these compositions give rise to harmful decomposition products during sponge production and thus pose an environmental problem. Patent Publication 7-122000 proposes a silicone rubber sponge composition comprising an oil-in water type emulsion consisting of dimethylpolysiloxane, an emulsifier, water, and a thickener. However, this composition does not readily give silicone rubber sponges with uniform cells. Further, since oil-in water type emulsions and silicone rubber compositions are basically immiscible, the process of evenly dispersing the silicone rubber composition throughout the silicone rubber composition is time consuming and throughput is poor.
It is an object of the present invention to provide a silicone rubber sponge composition that gives a silicone rubber sponge having fine, uniform cells, a silicone rubber sponge, and a process for production thereof.
The present invention relates to a silicone rubber sponge composition, a silicone rubber sponge, and a process for production thereof. The silicone rubber sponge composition comprises (A) 100 parts by weight organopolysiloxane gum described by average structural unit RaSiO(4xe2x88x92a)2, where R is a monovalent hydrocarbon group or haloalkyl and a is 1.8 to 2.3 and having a viscosity at 25xc2x0 C. of 1,000,000 mPaxc2x7s or above, (B) 1 to 400 parts by weight inorganic filler, (C) 0.01 to 50 parts by weight hollow thermoplastic resin particles, (D) 0.01 to 10 parts by weight water-in-oil emulsion with silicone oil as an oil phase, and (E) a curing agent in an amount sufficient to cure the composition.
A first embodiment of the present invention is a silicone rubber sponge composition comprising
(A) 100 parts by weight organopolysiloxane gum described by average structural unit RaSiO(4xe2x88x92a)/2, where R is a monovalent hydrocarbon group or haloalkyl and a is 1.8 to 2.3 and having a viscosity at 25xc2x0 C. of 1,000,000 mPaxc2x7s or above,
(B) 1 to 400 parts by weight inorganic filler,
(C) 0.01 to 50 parts by weight hollow thermoplastic resin particles,
(D) 0.01 to 10 parts by weight water-in-oil emulsion with silicone oil as an oil phase, and
(E) a curing agent in an amount sufficient to cure the composition.
A second embodiment of the present invention is a silicone rubber sponge produced by heat curing of the silicone rubber sponge composition. A third embodiment of the present invention is a process for production of a silicone rubber sponge composition comprising the steps of combining components (A) and (B) to produce a silicone rubber base compound and incorporating components (C), (D), and (E) into the silicone rubber base compound. A fourth embodiment of the present invention is a process for production of a silicone rubber sponge comprising the step of curing the present composition by heating to a temperature equal to or above the softening point of the thermoplastic resin of component (C).
Component (A) is the principal component of the present composition. It must have a viscosity at 25xc2x0 C. of 1,000,000 mPaxc2x7s or above and preferably 5,000,000 mnPaxc2x7s or above. Component (A) is a gum at normal temperature and has a Williams plasticity of 50 or greater, preferably 100 or greater, and more preferably 120 or greater. The degree of polymerization of component (A) is typically 3,000 to 20,000, with the weight-average molecular weight being 20xc3x97104 or above. The class of compounds known as organopolysiloxane gums used as the principle component in organic peroxide-curing millable compositions can be used for component (A). Component (A) consists of an organopolysiloxane gum described by average unit formula RaSiO(4xe2x88x92a)/2, where R is a monovalent hydrocarbon group or haloalkyl and a is 1.8 to 2.3. Monovalent hydrocarbon groups represented by R include alkyls such as methyl, ethyl, and propyl; alkenyls such as vinyl and allyl, cycloalkyls such as cyclohexyl; aralkyls such as xcex2-phenylethyl; and aryls such as phenyl and tolyl. Haloalkyl groups represented by R include 3,3,3-trifluoropropyl and 3-chloropropyl.
Where the curing agent consists of either an alkyl peroxide or a platinum catalyst used concomitantly with an organopolysiloxane containing silicon-bonded hydrogen atoms, the organopolysiloxane gum molecule must have at least two silicon-bonded alkenyls. Alkenyl here refers, for example, to vinyl, ally, propenyl, and hexenyl groups. The molecular structure of component (A) may be linear or linear containing branches. Component (A) may be a homopolymer, copolymer, or a blend of polymers. Specific examples of the siloxane units of component (A) are dimethylsiloxane, methylvinylsiloxane, methylphenylsiloxane, and (3,3,3-trifluoropropyl)methylsiloxane. Endgroups present on the molecular chain terminals of component (A) include, for example, trimethylsiloxy, dimethylvinylsiloxy, methylvinylhydroxysiloxy, and dimethylhydroxysiloxy groups. Examples of such organopolysiloxane gums include methylvinylpolysiloxane gum endblocked at both terminals with trimethylsiloxy groups, a copolymer gum of methylvinylsiloxane and dimethylsiloxane endblocked at both terminals with trimethylsiloxy groups, dimethylpolysiloxane gum endblocked at both terminals with dimethylvinylsiloxy groups, a copolymer gum of methylvinylsiloxane and dimethylsiloxane endblocked at both terminals with dimethylvinylsiloxy groups, a copolymer gum of methylvinylsiloxane and dirnethylsiloxane endblocked at both terminals with dimethylhydroxysiloxy groups, a copolymer gum of methylphenylsiloxane, methylvinylsiloxane and dimethylsiloxane endblocked at both terminals with methylvinylhydroxysiloxy groups, and a copolymer gum of (3,3,3-trifluoropropyl)methylsiloxane, methylvinylsiloxane and dimethylsiloxane that is endblocked at both terminals with methylvinylhydroxysiloxy groups.
Examples of the inorganic filler of component (B) are reinforcing fillers such as finely divided silica (e.g. dry process silica or wet process silica) and finely divided silica whose surfaces have been rendered hydrophobic through treatment with an organochlorosilane, organoalkoxysilane, hexaorganodisilazane, organosiloxane oligomer, or the like; and semi-reinforcing or extending fillers such as powdered quartz, diatomaceous earth, heavy calcium carbonate, light calcium carbonate, magnesium oxide, calcium silicate, mica, aluminum oxide, aluminum hydroxide, carbon black, and the like. In excessively large amounts component (B) is difficult to incorporate into component (A) and accordingly the preferred range is 1 to 400 parts by weight per 100 parts by weight of component (A), preferably from 1 to 100 parts by weight for the reinforcing fillers and from 1 to 150 parts by weight for the semi-reinforcing or extending fillers.
The hollow thermoplastic resin particles used for component (C) serve as nuclei for cell formation and also make the cell distribution uniform. An exemplary component (C) is a material consisting of thermoplastic resin shells having an inert gas enclosed therein. Thermoplastic resins include silicone resins, acrylic resins, and polycarbonate resins. In preferred practice, the thermoplastic resin will have a softening point of from 40 to 200xc2x0 C., and especially 60 to 180xc2x0 C. Inert gases include air, nitrogen gas, helium gas, and the like. Component (C) average particle size is preferably within the range of 0.1 to 500 xcexcm, and more preferably 1 to 50 xcexcm. Component (C) may be prepared, for example, by spraying an aqueous dispersion of a thermoplastic resin dissolved in a solvent from a spray nozzle into a heated air stream and evaporating the organic solvent while granulating the thermoplastic resin. Component (C) is included in amounts of 0.01 to 50 parts by weight, preferably 0.1 to 40 parts by weight, per 100 parts by weight of component (A).
Component (D), a water-in-oil emulsion with silicone oil as the oil component is a characterizing feature of the present composition and is essential in terms of creating fine, uniform cells. Component (D) can be prepared easily by dispersing a silicone oil in water using a surfactant.
The silicone oil forming the oil component of component (D) is an oligomer or polymer whose backbone is composed of diorganosiloxane units. It may be of liquid form, but is not limited to this kind. A typical example of the silicone oil is diorganosiloxanes described by general formula 
where R is a monovalent hydrocarbon group or haloalkyl and R1 is R or hydroxyl. Monovalent hydrocarbon groups represented by R include alkyls such as methyl, ethyl, propyl, butyl, pentyl, and hexyl; alkenyls such as vinyl, allyl, and hexenyl; cycloalkyls such as cyclohexyl; aralkyls such as P-phenylethyl; and aryls such as phenyl. Haloalkyl groups represented by R include 3-chloropropyl and 3,3,3-trifluoropropyl. Of the alkyl groups, methyl is preferred. The subscript n is an integer of 0 or greater. The silicone oil of component (D) can have a viscosity at 25xc2x0 C. of from 1 to 100,000 mPaxc2x7s and preferably from 10 to 100,000 mPaxc2x7s.
The surfactant used in component (D) can be any surfactant capable of producing a water-in-oil emulsion and the type is not critical provided that curing of the present composition is not hampered. Examples of surfactants capable of producing water-in-oil emulsions include diorganopolysiloxanes having polyoxyalkylene groups described by the following molecular formula on side chains 
where x and y are integers of 1 or greater, z is 0 or an integer of 1 or greater, A is a group described by general formula xe2x80x94(CH2)axe2x80x94Oxe2x80x94(C2H4O)p(C3H6O)qR2; where a is an integer from 1 to 3, p is an integer of 1 or greater, q is 0 or an integer of 1 or greater, R2 is hydrogen or a C1xe2x88x924 alkyl, and B is xe2x80x94(CH2)nxe2x80x94CH3 where n is an integer of 7 to 23. Other examples of surfactants useful in the present composition include dimethylpolysiloxanes having polyoxyalkylene groups of formula A, as described above, at their molecular chain terminals, polyoxyethylene sorbitol fatty acid esters, polyethylene glycol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, and other nonionic surfactants; as well as mixtures of the above polyoxyalkylene group-containing organopolysiloxanes with the above nonionic surfactants.
The water can be any type of xe2x80x9chighly purexe2x80x9d water such as distilled or deionized water or the like.
Component (D) is used in the present composition in an amount of from 0.01 to 10 parts by weight, preferably 0.1 to 5 parts by weight, per 100 parts by weight of component (A). With amounts of component (D) less than 0.01 parts by weight the silicone rubber sponge will not have a satisfactory expansion coefficient, while amounts exceeding 10 parts by weight may result in problems such as hindered curing.
Component (E), the curing agent, is an organic peroxide, or a platinum catalyst plus an organopolysiloxane containing silicon-bonded hydrogen. Examples of the former type, namely organic peroxides, include benzoyl peroxide, t-butyl perbenzoate, o-methyl benzoyl peroxide, p-methyl benzoyl peroxide, m-methyl benzoyl peroxide, dicumyl peroxide, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane. The amount of the organic peroxide is preferably 0.1 to 10 parts by weight per 100 parts by weight of component (A).
When component (E) is a platinum catalyst plus an organopolysiloxane containing silicon-bonded hydrogen, examples of the platinum catalyst are finely divided platinum, platinum black, chloroplatinic acid, alcohol-modified chloroplatinic acid, chloroplatinic acid olefin complexes, chloroplatinic acid/diketone complexes, and chloroplatinic acid/1,3-divinyltetramethyldisiloxane complexes. In preferred practice, the amount of platinum catalyst, expressed as metallic platinum, is from 0.1 to 500 ppm (weight basis) of the total composition. Organopolysiloxanes containing silicon-bonded hydrogen are crosslinking agents and in the presence of platinum catalysts react with the alkenyl groups in component (A) to cure the present compositions. Examples of organopolysiloxanes containing silicon-bonded hydrogen are methylhydriosiloxane endblocked at both terminals with trimethylsiloxy groups, a copolymer of methylhydriosiloxane and dimethylsiloxane endblocked at both terminals with trimethylsiloxy groups, a copolymer of methylhydriosiloxane and dimethylsiloxane endblocked at both terminals with dimethylhydriosiloxy groups, and tetramethyltetrahydriocyclotetrasiloxane. In preferred practice the amount of the organopolysiloxane containing silicon-bonded hydrogen will be such that the molar ratio of silicon-bonded hydrogen to the alkenyl groups in component (A) is 0.5:1 to 10:1. Compounds known in the art as agents for regulating the catalytic activity of platinum catalysts such as 1-ethynyl-cyclohexanol, 3-methyl-1-penten-3-ol, or benzotriazole, may be added as well.
The present composition comprises components (A) to (E) described hereinabove. Additives known in the art for inclusion in silicone rubber sponge compounds may be included as well, provided that the objects of the invention are not impaired thereby. Examples of such additives include heat stability agents such as iron oxide, cerium oxide, and fatty acid cerium salts; flame retardants such as manganese carbonate, zinc carbonate, and fumed titanium dioxide; pigments such as red iron oxide, titanium dioxide, and carbon black; and silicone oils such as dimethylsilicone oil and methylphenylsilicone oil.
The present composition can be easily prepared by evenly mixing components (A) to (E) plus any other ingredients that may be required. In preferred practice, component (A) will be premixed with any reinforcing fillers of component (B) to produce a silicone rubber base compound to which are then added components (C), (D), and (E). Where the reinforcing filler is a wet process silica or dry process silica that has not been treated to make it hydrophobic, it is preferable to prepare the silicone rubber base compound adding a plasticizer, such as a dimethylpolysiloxane oligomer endblocked at both terminals with silanol groups, or diphenylsilanediol. Examples of production equipment are kneader mixers, continuous kneader extruders, and other mixing or blending units.
Silicone rubber sponges may be produced from the present composition by heating to a temperature above the softening point of the thermoplastic resin of component (C) and curing. Silicone rubber sponges are formed by blowing and curing of the present composition. Silicone rubber sponges produced in this way have fine, uniform cells and excellent mechanical strength, making them useful as construction material airtight retaining gaskets, fire resistant gaskets, sealing materials, O-rings, and cushioning materials, as well as sheath materials for rollers in copiers and the like.